it depends what your goal is.
I even though I have 2 four stars ready to be champed and a couple more getting close I likely won't touch them until I build up all of my 2 and 3 stars.
the 2 stars get you covers for some 3 stars and the 3 stars can get you 4 star covers so they chain up.
if you are far enough along though, champing 3 stars and just working on them is a fine way to go, but I much prefer not selling 2 star covers thus I plan to have a full set of them and cycle them when I finish them since you only lose a couple thousand iso to cycle them and keep getting 3 star covers from them.
granted most of the 2 stars don't give great covers, and most of them that do are the ones that got lazy versions of them, but still